Electrolytic reduction of carbon dioxide to yield multicarbon products
The paper I'll discuss in this post is this one: CO2 electrolysis to multicarbon products at activities greater than 1 A cm^(?2) (F. Pelayo García de Arque al, Science, Vol. 367, Issue 6478, pp. 661-666).
There is a growing, and frankly delusional, belief that electricity is "green," i.e. that is inherently sustainable and clean. This is pure nonsense. First of all, except possibly in the case of lightening which is not utilized to charge Tesla cars, electricity is not primary energy. Since it must be made from a source of primary energy, it is therefore, by appeal to the inviolable laws of thermodynamics, a degraded form of energy: Whenever electricity is generated, irrespective of how it is generated, some energy is lost to entropy. Moreover, electricity must be either used when it is generated, or stored, with additional thermodynamic loses, as chemical energy, usually in the form of batteries, batteries representing a rising threat to the environment whether we get it or not.
This pernicious fantasy which helping to push the planetary ecosystem over an abyss that we cannot even remotely imagine, is based on an unsubstantiated bit of nonsense that pretends, in Trumpian contempt for reality, that electricity is, or soon will be, generated by another pernicious, but popular, fantasy, so called "renewable energy." In turn, so called "renewable energy" is not clean and is not sustainable. Even it it were, despite all the cheering, it is largely ineffective at producing energy, and the two trillion dollars per decade squandered on it - the current rate - will not change that fact. In this century the growth in the use of so called "renewable energy" has been dwarfed by the increases in the use of dangerous coal, the use of dangerous petroleum, and the use of dangerous natural gas. This is true in every area of energy use, but it is equally so in the case of electricity generation.
The International Energy Agency puts out, every year, along with the World Energy Outlook, to which I often refer, a document called "Electricity Information." Here is a link to the 2019 Edition: Electricity Information 2019: Overview
Here is a graphic from it:
Coal remained, as of 2017, the world's largest source of primary energy used to generate electricity.
A similar graphic, in the same document, on the same page, shows the actual primary energy generated by all fuels:
Both graphics show how successful, for all the cheering, and the trillions of dollars thrown at it, so called "renewable energy" has been in displacing dangerous fossil fuels, which is to say not all. None of this experimental data, however, will prevent advocates of this scheme to turn our remaining (and vanishing) wilderness areas into industrial parks for wind turbine and solar farms, for no meaningful result. The use of dangerous fossil fuels, not just to produce electricity, but for all purposes, is rising, not falling.
On page II.4 of the EIA Electricity Information Document, is a rather large table (Table 1.0 of section II of the report) relating to energy use in general and what proportions of it involves the generation electricity.
The table contains some interesting information about the use of coal for purposes other than generating electricity or heat, with three categories, steel production, non-ferous metals, and non-metal mineral processing (probably mostly representing concrete production) - the demand for all three will rise in the worst case scenario, where the world continues to expand so called "renewable energy" - amounted to about 23.3 exajoules of energy. Other ancillary uses for coal also require prodigious amounts of this rather unsustainable material.
I won't reproduce the table here, but note that, with some calculation - which I have done - one can obtain the thermodynamic efficiency of all the major forms of energy utilized to generate electricity, as well what fraction of the total generation derives from a particular source of primary energy. The following table is the result of my calculations from Table 1.0 of section II.
To the extent that CHP, combined heat and power, is present, suggests that, at least in Winter months, not all of the entropy (heat) losses are truly wasted, but the fact is that overall, for all types of plants that produce electricity - clearly these tables ignore transmission losses, since otherwise hydroelectricity would not be recorded as 100% efficient - the energy efficiency is 42.31%.
(In fact, no system for generating electricity can actually work at 100% efficiency, as is stated in this table for hydroelectricity. The "100%" figure ignores that the system is inefficient when the primary energy to drive the hydroelectric turbines is actually gravitational energy associated with the mass of water that falls through the turbine. It is probably too painful to calculate, and so we have this somewhat disingenuous 100% figure. With this in mind, we should not that the 42.31% figure is clearly too high, since hydroelectricity produces about 15% of the world's electricity. In any case, we are almost fresh out of major rivers to destroy with hydroelectric plants. The less than 100% efficiency for so called "renewable energy" is difficult to explain in the same terms - except for geothermal - and may reflect the fact that it is often required to dump so called "renewable energy" because of saturated grids, where there is too much wind and solar with the result that every energy system on that grid, including so called "renewable energy," is economically useless to the owners of the plants. It also may reflect the use of batteries. Who cares? So called "renewable energy" is best at generating not energy, but rather at generating evidence of its uselessness.)
The point is, overall, that electricity is a degraded form of primary energy. The highest efficiency for a thermal system, belongs to dangerous natural gas fuels, a point to which I will return briefly in the summary of this post, since although dangerous natural gas is in no way a sustainable or acceptable fuel - it must be phased out in its entirety - one can certainly learn from how it is used to achieve higher thermodynamic efficiency than other systems.
Therefore, when one stores electricity in a chemical form - a practice in itself that is never 100% efficient, one further degrades the energy efficiency of the system. That is true for batteries, and it is true for the electrochemical reduction of carbon dioxide described in the paper referenced at the outset of this post.
The paper begins with the typical rote obeisance - found in almost all energy storage papers these days - to so called "renewable energy."
In these reactions, water-based electrolytes act both as a proton source and as the ion conductive medium (10). However, the solubility of these gases in water is limited, leading to constrained gas diffusion as gas molecules collide or react with their environment (11). The diffusion length of CO2 can be as low as tens of nanometers in alkaline aqueous environments (12). This has limited the productivity of catalysts in aqueous cells to current densities in the range of tens of milliamperes per square centimeter due to mass transport (13–16).
In a gas-phase electrolyzer, catalyst layers are deposited onto hydrophobic gas-diffusion layers so that gas reactants need to diffuse only short distances to reach electroactive sites on the catalyst surface (Fig. 1A) (17–19). Gas reactant diffusion in the catalyst layer becomes the mass transport–limiting step in the cathode, as observed in the oxygen reduction reaction (ORR) in fuel cells. To improve ORR performance, fuel-cell catalyst layers are designed to balance hydrophobicity to help expel water and hydrophilicity to maintain sufficient ion conductivity.
In contrast with oxygen reduction, which generates water as a product, CO2 reduction requires water as a proton source for hydrocarbon production. Thus, the catalyst layer is hydrophilic and fully hydrated during the reaction. In this configuration, CO2 electrochemical reactions occur within a gas-liquid-solid three-phase reaction interface (Fig. 1B) (20). This volume, in which gaseous reactants and electrolytes coexist at catalyst electroactive sites, decays rapidly into the electrolyte, particularly at the high pH used in alkaline electrolysis. The decay is further increased at high current densities because of local OH? generation (21). A large fraction of the catalyst is in contact with electrolyte in which CO2 availability is limited by its solubility (<2 mM at pH 15). Because hydrogen evolution is a competing reaction with CO2 reduction in a similar applied potential range, the large fraction of catalyst surface area exposed to CO2-depleted electrolyte promotes undesired H2 generation (Fig. 1C). Whereas recent advances in gas-phase CO2 reduction have led to partial current densities for CO2 reduction of ?100 mA cm?2 (12, 22, 23), other liquid-phase electrochemical technologies such as water electrolysis achieve multi-amperes per square centimeter (24, 25).
Figure 1:
The caption:
(A) Flow-cell schematic. Reactant gases are fed through the back of a gas diffusion–electrode catalyst, facing an aqueous electrolyte. An anion-exchange membrane (AEM) facilitates OH? transport from cathode to anode. GDL, gas-diffusion layer. (B) In a gas-diffusion electrode (GDE), catalysts are deposited onto a hydrophobic support from which gas reactants [G] diffuse. (C) The volume in which gas reactants, active sites, and water and ions coexist determines the maximum available current for gas electrolysis. Catalyst regions with limited reactant concentration promote by-product reactions such as hydrogen evolution. (D) When gas and electrolyte (water and ion source) transport is decoupled, the three-phase reaction interface can be extended so that all electrons participate in the desired electrochemical reaction. (E and F) Modeled gas reactant availability along the catalyst’s surface for standard (E) and decoupled (F) gas transport into a 5 M KOH electrolyte, assuming an in-plane laminar gas diffusivity of D‖/DKOH = 1000 for the latter, where D‖ is gas diffusivity parallel to catalyst surface. Depending on the gas diffusivity within the gas transport channel, gas availability dramatically increases. L‖, distance parallel to catalyst surface; L?, distance perpendicular to catalyst surface. (G) Modeled maximum available current density for CO2 reduction. D/DKOH manipulation enables entrance into the >1–A cm?2 regime for CO2R. See methods for details on gas transport and reaction simulations.
This is a gas phase system, which involves gas phase water (steam), and thus involves high temperatures which can only be provided by wind and solar so called "renewable energy" via an electricity intermediate, again, thermodynamically degraded energy:
Here, we present a hybrid catalyst design that, by decoupling gas, ion, and electron transport, enables efficient CO2 and CO gas-phase electrolysis at current densities in the >1–A cm?2 regime to generate multicarbon products. We exploit an ionomer layer that, with hydrophobic and hydrophilic functionalities, assembles into a morphology with differentiated domains that favor gas and ion transport routes, conformally, over the metal surface: Gas transport is promoted through a side chain of hydrophobic domains, leading to extended gas diffusion, whereas water uptake and ion transport occur through hydrated hydrophilic domains (Fig. 1D). As a result, the reaction interface at which these three components come together—gaseous reactants, ions, and electrons—all at catalytically active sites, is increased from the submicrometer regime to the several micrometer length scale.
The system that the authors design is a functionalized type Nafion based system of electrodes. Nafion is a fluoropolymer. In general, fluoropolymers, while extremely useful, are a source of the intractable fluoroalkane (PFOS, PFOA) contamination issue that has become recently an area of increasing environmental concern.
Figure 2 of the paper:
The caption:
(A) Schematic of metal catalyst deposited onto a PTFE hydrophobic fiber support. A flat ionomer layer conformally coats the metal. (B) Perfluorinated ionomers such as Nafion exhibit differentiated hydrophilic and hydrophobic characteristics endowed by –SO3– and –CF2 functionalities, respectively. Laminar Nafion arrangements have been reported depending on its thickness and substrate (37, 40). (C and D) SEM images of ionomer-coated copper catalysts. (E to G) Cryo-microtomed TEM cross-sections of catalyst and ionomer revealing a laminar conformal overcoating. (H) WAXS spectra for reference and ionomer-modified catalysts. These reveal features at 1, 1.28, and 2 A?1, associated with various PFSA and PTFE-support phases. (I) Raman spectra of reference and ionomer-modified catalysts revealing distinctive features of ionomer –CF2 and –SO3? groups (table S5).
Figure 3 shows limiting currents for these systems:
In the caption, "RR" stands for "reduction reaction" and "ORR" stands for "oxygen reduction reaction," CORR for "carbon monoxide reduction reaction" and CO2RR to "carbon dioxide reduction reaction." CIPH refers to "catalyst:ionomer planar heterojunction" which refers to the type of electrodes the authors have developed in this paper.
The caption:
(A) ORR showing a 30–mA cm?2 limiting current (Jlim) for Ag reference catalysts as opposed to 250 mA cm?2 for a CIPH configuration. RHE, reversible hydrogen electrode. (B) For CO2RR, standard Ag catalysts yield a Jlim of ?54 mA cm?2 (remaining current used for hydrogen evolution). This is in stark contrast with CIPH samples, which retain a FE above 85% for CO2 reduction (CO2R) to CO up to ?500 mA cm?2. (C) This trend is maintained for Cu CIPH catalysts and hydrocarbon production: Jlim toward ethylene (dominant product) is 50 mA cm?2 at ?0.7 V versus RHE for bare Cu but increases beyond 0.5 A cm?2 for CIPH (peak FE of 61% at 835 mA cm?2). (D) For CO reduction (COR), Jlim ? 64 mA cm?2 for standard Cu, whereas CIPH achieves a maximum 340–mA cm?2 current for the same reaction; H2 by-product generation is restrained below 15% FE at all currents. (E and F) Partial pressure COR studies in CO|N2 mixes for CIPH (E) and standard (F) catalyst show that only at partial pressures below 60% is Jlim observed for CIPH, whereas a sharp, steady decrease is observed for reference samples. At all partial pressures, CIPH exhibits an order of magnitude larger Jlim. Both reference and CIPH samples exhibit comparable resistance and double-layer capacitance. Electrochemical experiments were carried out in 5 M KOH electrolyte with a 50–cm3 min?1 CO or CO2 feedstock (in the case of 100% partial pressure).
The unusual, and important point about this technology is the fact that it produces ethylene, which is the monomer utilized to make the polymer polyethylene, and is also a useful intermediate for the production of many other types of polymers and chemicals, including, but certainly not limited to, ethanol. As such, the technology allows for the elimination of the use of dangerous fossil fuels in the manufacture of this intermediate, thus eliminating their contribution to climate change - which is not to say we give a rat's ass about climate change; clearly we don't. If we did, we'd stop carrying on about so called "renewable energy" - since it has been experimentally determined to be useless at addressing climate change.
Some interesting stuff about the catalyst morphology is shown in figure 4:
The caption:
(A) Schematic representation of metal-ionomer bulk heterojunction catalysts on a PTFE support. (B) Cross-sectional SEM of the CIBH catalyst. (C and D) TEM image of a cryo-microtomed CIBH (C) and elemental mapping of Cu and C revealing CIBH nanomorphology (D). (E) Partial current density for total CO2RR reactions, with C2+ and C2H4 at maximum cathodic energy efficiency. The total CO2R current saturates at 1.3 A cm?2 before cathodic energy efficiency drops for CIBH thicknesses beyond 6 ?m. CIBH samples achieve more than a sixfold increase in partial current density at cathodic energy efficiencies >40% (fig. S30). Each sample and operating condition ran for at least 30 min. (F) Performance statistics of the highest partial current configuration for eight Cu CIBH catalysts. The box plot corresponds to Q1 to Q3 interquartile range, median, and average. The error bar represents ?5.4 standard deviations. EE1/2, half-cell (cathodic) energy efficiency. (G) Performance of the best CIBH catalyst in an ultraslim flow cell consisting of a 3-mm-wide catholyte channel. A full-cell energy efficiency of 20% for C2+ products is estimated at 1.1–A cm?2 operating current. All CIBH electrochemical experiments were carried out in 7 M KOH with a 50–cm3 min?1 CO2 feedstock.
The authors conclude their article with a discussion of efficiency and of course, evocation of the wonder word "renewable:"
The product distribution for optimal CIBH catalysts at different current densities in 7 M KOH electrolyte reveals that H2 generation remains below 10% from 0.2 to 1.5 A cm?2 (fig. S29). At the highest current operation, optimized catalysts exhibited a maximum productivity toward ethylene with a FE in the 65 to 75% range, a peak partial current density of 1.34 A cm?2 at a cathodic energy efficiency of 46 ± 3% (Fig. 4F and figs. S31 and S32). We implemented the best CIBH catalyst in an ultraslim flow cell (with no reference electrode and a minimized catholyte channel of ?3 mm, with water oxidized at a Ni foam anode), leading to an estimated full-cell energy efficiency toward C2+ products of 20% at 1.1 A cm?2 without the benefit of iR compensation (i, current; R, resistance) (Fig. 4G). CIBH catalyst current and FE remained stable over the course of a 60-hour initial study implemented in a membrane electrode assembly configuration (fig. S33).
Although CO2 reduction kinetics improve with increasing temperature, alkaline electrolyzers manifest worsened CO2 availability as temperature increases, and this fact curtails reaction productivity. We explored the effect of temperature on planar CIPH metal:ionomer catalysts and observed that CIPH catalysts require lower overpotentials to attain similar FE, in contrast with planar reference catalysts (fig. S34), when operated at 60°C. This effect translates into 3D CIBH catalysts, which show improved performance arising from the combination of accelerated CO2 reduction kinetics and extended mass transport through the ionomer layer with increasing temperature (fig. S35). As a result, CIBH catalysts achieve ?1 V reduced overpotential and more than a 50% increase in C2 productivity when operated at industrial electrolyzer-relevant temperatures of 60°C in a full-cell configuration, compared with the case of room temperature operation (fig. S36).
The phenomena described herein showcase catalyst design principles that are not constrained by prior gas-ion-electron transport restrictions. The CIBH catalyst paves the way to the realization of renewable electrochemistry for hydrocarbon production at operating currents needed for industrial applications, as has been achieved with syngas for solid oxide electrolyzers (48, 49).
The cathodic efficiency comes from the efficiency of converting electricity - thermodynamically degraded energy - into these products. It does not include the thermodynamic cost of separating the four products, although the separation of ethylene gas is something of a no-brainer.
I don't believe for a New York second that this technology is applicable in an economically or environmentally acceptable fashion to so called "renewable energy." The economics of any production system depends very much on its capacity utilization. Since the capacity utilization of the mass intensive so called "renewable energy" industry is low, and decoupled intrinsically from demand loads, it follows that the capacity utilization of these systems in so called "renewable energy" systems will be even lower. Thus for long periods, the capacity built to construct such a system will be generating zero value to an investor, and moreover, the return on investment will be unpredictable.
Nevertheless may be useful in limited applications, depending on the design of a power grid, for continuous operation systems. The power grid in most places is uneven. Generally peak loads on grids occurs in the early evenings and late afternoons. If the output of so called "renewable energy" systems in places where they have foolishly represent a large portion of the grid sources is momentarily high, all power sources return nothing to their investors, and for systems that are reliable but uneconomical at random points, to be available for those times when the sun is not shining and the wind not blowing, to survive they must recover their costs in the periods in which they are required to operate to prevent blackouts. This is why Denmark and Germany have the highest retail electricity prices in the OECD.
Nuclear power plants operate best at close to 100% capacity utilization. As the table above shows, they operate at unacceptably low thermodynamic efficiency, about 33%. The reason for this is that the basic technology under which they were built was to produce electricity; they were designed to be coal plants without the coal. They operate overwhelmingly (with some exceptions) on Rankine (steam) cycles.
We know from the much discussed events at Three Mile Island and Fukushima - events that garner far more attention than 70 million deaths from air pollution every decade - (and of course from engineering and science courses, were we interested in taking them) that nuclear fuels can produce heat that is much higher than the boiling point of water. It is possible to use these high temperatures to skip the thermodynamically degraded electricity intermediate to increase thermodynamic efficiency by simply proceeding directly to chemical storage using heat. Thermochemical cycles to do this are well known and widely studied. In fact, they can be operated synergistically with electricity production, for example using a modified Allam cycle, a heat engine cycle I discussed elsewhere in this space.
It is the use of two heat engine cycles in tandem that accounts for the high thermodynamic efficiency of gas plants: They have Brayton cycles (of which Allam cycles are a subset) coupled to Rankine cycles, using the waste heat from the high temperature cycle, the Brayton cycle, to drive the lower temperature cycle, the Rankine cycle. Their are other possibilities to go beyond these, thermochemical cycles (which represent stored energy as fuels or materials) coupled to Brayton cycles (with a carbon dioxide working fluid) driving a Rankine steam cycle, with high efficiency thermoelectric devices and or Stirling engines being possibly added in the temperature reduction line. This would have the added advantage of reducing the water demand for cooling of such plants, although it is also possible to put a desalination scheme somewhere in the line. There is a world of better ways to do things.
All of these high temperature high efficiency schemes are dependent on access to continuous reliable energy that is also clean and sustainable. This limits the choice to one source of primary energy, nuclear energy.
As for the electrochemical reduction of carbon dioxide, it might be utilized, in the limited setting of preventing the waste associated with spinning reserve, depending on the economic cost of the entire system and its relationship to capacity utilization. Spinning reserve is the amount of power that is generated to cover fast and short term and unexpected surges in demand on the system without producing brown outs. To the extent that this energy, when not in demand, is utilized to drive the electrochemical reduction of carbon dioxide to ethylene, it's certainly worth consideration.
I wish you a pleasant and productive workweek.
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